This invention relates to an apparatus for counting particles entrained in a fluid medium flow and, in particular, to an apparatus adapted to count and to resolve particles having sizes that are small as compared to the illuminating wavelength and/or have an index of refraction close to that of the medium.
Subject matter disclosed herein is disclosed in the copending application of S. D. Abbott et al. titled "Particle Reagent Size Distribution Measurements for Immunoassay", Ser. No. 474,483, U.S. Pat. No. 4,521,521 and the copending application of S. D. Abbott titled "Particle Counting System For A Fractionating Device," Ser. No. 474,481, both filed contemporaneously herewith.
Particle size analysis and counting instruments based upon light scattering techniques adapted to count particles entrained in aerosol and translucent fluid medium (primarily liquid) flows are known. Exemplary of such apparatus is that sold by Polytec Optronics, Incorporated of El Toro, Calif. as model HC15. It is often of advantage to obtain a histogram by count or other representation of the number of particles having sizes that are small as compared to the illuminating wavelength and/or that have an index of refraction close to the index of refraction of the medium. However, such apparatus as is commercially available is not believed able to provide quantitative measurements of the number of particles having such sizes or indices of refraction.
The Mie Theory relates to the radiation scattering properties of particles that are small as compared to the free space wavelength of the incident radiation. See, generally, Kerker, "The Scattering of Light and Other Electromagnetic Radiation", Academic Press, Inc. (1969). For purposes of this application, the term "size parameter" of a particle may be assigned the character ".alpha." and is defined by Mie as follows: ##EQU1## where m.sub.2 is the index of refraction of the medium in which the particle is entrained,
.alpha. is the radius of the particle, and PA1 .lambda..sub.o is the free space wavelength of the incident radiation on the particle.
The term "relative refractive index" is assigned the character "m" and may be defined as follows: ##EQU2## where m.sub.1 is the index of refraction of the particle.
The term "sensitivity limit of detection" of a particle is assigned the character "S" and may be defined as follows: EQU S=.vertline.(m-1).vertline..alpha. (3)
It is believed advantageous to provide a particle counting apparatus adapted to count particles each having an index of refraction close to that of the medium in which it is entrained and a radius on the order of 0.05 micrometers. In terms of The Mie Theory, for incident radiation with free space wavelength on the order of 0.633 micrometers, m.sub.1 on the order of 1.59 and m.sub.2 on the order of 1.33, such a particle has a sensitivity limit of detection S of at least 0.129.